What Is a Sash Lock? Types, Sizes and Security Explained

At Trade Door Handles, we've specialised in supplying architectural ironmongery for decades, stocking a huge range of locks and latches for doors across the UK — from bathroom locks and tubular latches to 2, 3 and 5 lever sash locks and deadlocks for commercial and domestic properties. It's one of the questions our trade counter and customer service team hear most often: what exactly is a sash lock, and how do you choose the right one for your door?

This guide explains everything you need to know, including what they are, the difference between lever types, standard sizes, security ratings, and where they should and shouldn't be fitted.

Supplied by Trade Door Handles, trusted UK supplier of architectural ironmongery since 1976, with quick nationwide delivery.

A sash lock is a type of mortice lock fitted into a pocket cut into the edge of a door, combining two separate mechanisms within a single lock case: a sprung latch bolt and a key-operated deadbolt. The latch keeps the door held shut day-to-day, operated by turning a door handle, while the deadbolt is engaged separately using a key for added security when the property is unoccupied or overnight.

Because a sash lock includes a latch, it always needs to be fitted alongside a set of door handles or lever furniture — take a look at our Locking Door Handles on Backplates, designed to work with 2, 3 and 5 lever sash locks — unlike a deadlock, which is operated by key alone and doesn't require handles to function.

The name has nothing to do with sash windows, despite the shared word — it comes from older ironmongery terminology where "sash" simply described the moving latch bolt inside the lock case. Inside the body, a spindle passes through a follower connecting the two handles, and turning either handle rotates the follower to retract the sprung latch. The deadbolt sits entirely separately within the same case and only moves when the key is turned, so the two mechanisms never interfere with one another. This dual-mechanism design is exactly why the lock type is so widely specified across the UK: one product gives you everyday convenience through the latch, plus proper key security through the deadbolt, without needing two separate locks fitted to the same door.

The easiest way to understand a sash lock is to compare it with a mortice deadlock:

• A deadlock contains only a key-operated deadbolt. It has no latch, so the door won't hold itself shut — it must be locked by key each time.
• A sash lock contains both a latch and a deadbolt. The latch holds the door closed automatically, while the deadbolt adds extra security when locked.

In practice, they are generally specified wherever a door needs to close and hold itself shut under everyday use, while deadlocks are more commonly fitted as a secondary lock alongside a nightlatch or another locking mechanism.

You'll often see this type of lock described as a mortice sash lock, and the two terms mean exactly the same thing. "Mortice" refers to the pocket cut into the door edge that houses the lock body, keeping the mechanism hidden and the door edge flush and neat. You can browse our full range of Mortice Locks and Latches to see how it fits within the wider mortice lock family.

Sash locks are also available with a euro cylinder rather than a traditional key and levers — known as euro profile sash locks — which allow the cylinder to be replaced independently if a key is lost, without changing the entire lock body. Take a look at our Euro Profile Sash Locks collection for this option, paired with matching Euro Profile Door Handles on Backplates to complete the set.

You may also come across oval profile versions, which work on exactly the same principle but use an oval-shaped cylinder rather than a round euro cylinder. Oval profile hardware is less common in the UK than euro profile, but it's still specified on certain older doors and some continental-style ironmongery, and the cylinder can still be replaced independently of the lock body if a key is lost or a resident moves out. If you're not sure which profile your existing lock uses, look at the shape of the keyway on the forend: a small round opening points to euro profile, an oval-shaped opening points to oval profile, and a traditional keyhole shape — wider at the top with a narrow slot below — means you have a standard lever-operated lock rather than a cylinder version at all. Getting this right before ordering avoids buying a replacement that won't fit the existing cut-out in the door.

Sash locks are classified by the number of levers inside the lock mechanism. More levers mean more possible key combinations, which increases both security and resistance to picking.

• 2 lever sash locks — the lowest security option, generally suited to low-risk internal doors such as cupboards and utility doors.
• 3 lever sash locks — a step up in security, commonly used on internal doors including bedrooms, studies and offices.
• 5 lever sash locks — the highest security standard, typically specified for external and front doors, and the only lever count eligible for BS3621 certification.

2 Lever Sash Locks offer basic key security and suit internal doors where a full locking function isn't essential, rather than anywhere requiring insurance-rated security. 3 Lever Sash Locks step this up with more key variations, and are a popular choice for bedrooms, studies and internal office doors — though, like the 2 lever option, they aren't generally accepted as insurance-rated for external doors.

5 Lever Sash Locks are the standard specification for external doors. When manufactured to meet BS3621, they offer the level of security most UK home insurers require on a front or back door.

Sash lock sizing is described by the case size, which relates to the backset — the distance from the edge of the door to the centre of the keyhole. In the UK, sash locks are most commonly available in 64mm (2.5") and 76mm (3") case sizes, with some manufacturers also offering additional sizes for non-standard doors.

The 64mm (2.5") case size is the most widely fitted sash lock size on standard UK timber doors, offering a good balance of handle position and lock strength. Larger 76mm (3") cases are also common, particularly on wider doors or where a specific handle centre measurement is needed to match existing hardware.

If you're replacing an existing sash lock, take the following measurements before ordering:

1. Backset — the distance from the edge of the door to the centre of the keyhole or spindle.
2. Case size — the overall length of the lock body, usually 64mm or 76mm.
3. Faceplate dimensions — the length and width of the plate visible on the door edge.
4. Handle centres — the distance between the spindle hole and the keyhole, so your new handles line up correctly.
5. Fixing centres — the distance between the screw holes, to confirm the faceplate fits your existing mortice pocket.

Taking accurate measurements first makes it far easier to find a straightforward like-for-like replacement without having to re-cut the door.

Sash locks suit both internal and external timber doors, provided a set of handles is fitted alongside them. They're one of the most commonly specified locks for front and back doors, as well as internal doors where day-to-day latching and occasional key security are both needed. It's worth noting that a traditional sash lock is designed for timber doors with a cut mortice pocket; composite and uPVC doors typically use a multipoint locking system operated by a euro cylinder instead, so always check the door construction before ordering.

Only sash locks that have been specifically tested and certified as part of a fire door assembly should be used on fire doors. A standard sash lock that hasn't been fire tested can compromise the integrity of the door in the event of a fire and may invalidate the door's certification. If you're specifying hardware for a fire door, always check the lock has supporting fire test evidence for the exact door construction being used.

Sash locks are widely used on guest bedroom doors in hotels, guest houses, B&Bs and holiday lets. In many modern properties, it's now recommended practice to fit a euro profile sash lock with a euro cylinder that has a thumbturn on the inside, rather than one that needs a key to open from within. This means guests can exit quickly in an emergency without having to search for a key first, giving an easy, unobstructed way out. Licensed premises such as HMOs and holiday accommodation often have specific fire safety and means-of-escape requirements set by local authorities or fire risk assessments, so it's always worth checking these requirements with your fire risk assessor or local authority before specifying locks for guest-facing doors in a commercial setting.

BS3621 is the British Standard for thief-resistant lock assemblies, and it's the certification most UK home insurers look for on external doors. A BS3621 kite-marked lock has been independently tested for:

• Bolt strength and resistance to forcing
• A minimum number of key variations, to reduce the risk of duplicate keys
• Resistance to picking and manipulation
• Resistance to physical attack, including drilling and sawing

No. Not every 5 lever lock carries BS3621 certification — the lever count alone doesn't guarantee it meets the standard. Always check for the BS3621 kite mark specifically if your insurance policy requires a certified lock, rather than assuming any 5 lever product will qualify.

Standard 5 lever locks can often be supplied as a key-alike set, allowing multiple doors to be opened using the same key — useful for properties with several external doors. Spare keys can usually be cut from the original by a locksmith or key-cutting service, although some high-security or patented key systems restrict duplication to authorised suppliers only. If you need multiple sash locks keyed alike, it's best to order them together as a set rather than trying to match keys afterwards.

As one of the UK's largest suppliers of architectural ironmongery, we stock sash locks from many of the biggest names in the industry, including Carlisle Brass, From The Anvil, Zoo Hardware, Frelan Hardware, ERA Fortress and Union Strongbolt, alongside other trusted manufacturers within our wider range. Whether you need a budget-friendly option for an internal door or a BS3621 certified lock for a front entrance, our team can help you specify the right brand and finish for your project. Finishes typically include polished brass, satin chrome, satin brass, black and pewter, so it's usually straightforward to match new hardware to existing handles, hinges and other ironmongery already fitted to the door.

Customers often ask us to help specify the right lock rather than simply pick one off the shelf, and the answer usually comes down to how the door is actually used. A busy front door on a family home behaves very differently to an internal study door or a uPVC back door on a multipoint system, so it's worth weighing up the three most common options side by side before ordering.

• Sash lock — best where a door needs to latch shut automatically and be locked with a key when needed. Requires handles.
• Deadlock — best as a secondary, key-only lock with no latch function, often paired with a nightlatch.
• Euro cylinder lock — best for uPVC, composite and some timber doors using multipoint locking systems, with the advantage of a replaceable cylinder.

There isn't a single "best" option — the right choice depends on the door construction, how the door is used day-to-day, and the level of security required.